1. Field of the Invention
This invention relates to forming toric contact lenses and more specifically to a lens forming machine and method that use a single-point flexure mechanism in combination with a low force, large stroke range actuator to provide the high speed cutting tool motion necessary to form a toric curve.
2. Description of the Related Art
A toric surface is characterized by a pair of orthogonal curvatures with different radii, and is typically formed by superimposing a toric deviation upon a spherical base curve. One type of contact lens forming machine, known as a "fast tool servo," includes a spindle that holds a lens blank in the X-Y plane and rotates it around its spindle axis. The spindle is mounted on a Z-axis slide that moves along the spindle axis. A single-point cutting tool is mounted on an X-axis slide that moves perpendicular to the lens blank.
To form the contact lens' base curve, the rotating lens blank is moved in the Z direction against the cutting tool. The tool is first moved progressively outward in the X direction away from the blank as the spindle moves in the Z direction until the tool reaches the middle of the blank, and is then moved progressively back inward towards the lens blank to complete the base curvature as the spindle and blank continue to pass in the Z direction. The force of the rotating lens tends to push the tool back and off-axis thereby distorting the surface. Therefore, it is very important that the cutting tool resists the motion of the lens blank and maintains its off-axis rigidity.
To superimpose a toric deviation on the base curve, the cutting tool is controlled by a piezoelectric crystal to make a series of small back-and-forth oscillations along the Z-axis during the formation of the lens's base curve. The oscillations are coordinated with the rotation of the lens blank so that the cutting tool makes one inward oscillation and one outward oscillation for every 180 degrees of lens blank rotation. This forms a pair of orthogonal toric lens curvatures with different radii.
The piezoelectric crystal produces the oscillations in response to a very high voltage that causes the crystal to shrink and expand. Because the piezoelectric crystal is very stiff, it produces a lot of force at the tip of the cutting tool so that the tool resists the motion/rotation of the lens blank. This same property restricts the motion of the crystal to approximately one one-thousandth of its length, which limits the stroke range of the cutting tool. Thus, to achieve the necessary stroke range the crystal must be very large. This is expensive and causes mounting problems. Furthermore, the very high voltages are dangerous and produce a lot of waste heat.
Wei Xu and Tim King, "Flexure hinges for piezoactuator displacement amplifiers: flexibility, accuracy, and stress considerations, " Precision Engineering 19, pp. 4-10, 1996 describe flexure hinges that sacrifice force in order to amplify the change in length of the piezoelectric crystal and increase the tool's stroke range. To maintain sufficient force to resist lens blank motion, the stroke range remains limited. Flexure hinge topologies typically combine three basic amplifying elements; simple lever, bridge, and four-bar linkage amplifiers.
As Xu states "The simple lever flexure-hinged displacement amplifier can produce large output displacements. Because the movement is arcuate, it may not, however be suitable for some high-precision applications requiring linear motion." Because the flexure hinge amplifies stroke range, any arcuate movement would be amplified at the tip of the cutting tool and introduce significant distortion into the lens. As a result, compound amplifiers are used to provide linear motion, but the compound amplifiers are expensive, complicated and sacrifice off-axis rigidity.